Some first approximation may be obtained from the data collected by Rind & Bauserman 1998. They consider retrospective college studies about the reaction of children. That's not the same - a relation may be valued negatively because of a lot of other reasons (i.e. reaction of environment, own failures) - but it may be a good lower estimate. It seems highly probable that sexual coercion or rape will be described as a negative experience.
In the studies considered by Rind & Bauserman, only a minority of young men react negatively: 38% (Finkelhor 1979), 30% (Goldman & Goldman 1988), 46% (Landis, 1956), 45% (O'Neill, 1990) und 8% (Schulz & Jones, 1983). On the other hand, young women in these studies usually react negatively: 66%, 68%, 76%, 82% und 52%. These results are in agreement with Baker & Duncan (1985) 37% negative, 63% not negative for men, 64% negativ, 36% not negative for women.
Information based on court cases was considered by Virkkunen 1981, who has collected evidence for the participation of the "victim" in various studies, looking for evidence like repeated visits. Because unwanted sexual contacts are much more likely to be reported, this may be used again only as a lower estimate. Again, an essential part of the cases is volitional. For example, among "homosexual offenders vs. children" Gebhard et al. (1965) found that 70% of the offenders said the boy had encouraged the sexual advances, or had been passive, which corresponded with the official records. In another 19% of the crimes there was agreement on the fact that the boys had resisted.
That means, the majority of sexual relations of boys with adults are volitional. The part of volitional relations of girls is lower, but nonetheless essential. The estimate of at least one third of all cases shows that volitional relations of girls cannot be ignored as a rare exception.